Mikki Osterloo
The growing season water balance for a watershed located in southwestern Montana
BSES Senior Research Project 2002

Abstract
Irrigation has long been a problem for many areas in the western states, and southwestern Montana is not an exception. Water is vital to the survival of the communities in southwestern Montana. Without ample irrigation most crops would be lost. This study will analyze the water balance to find how much water is needed and how much is being used. Because of complex water-rights issues, most farmers have been using the same amounts of water for generations without concern for how much water is needed for efficient crop production. Water in the region is usually thought of as more precious than gold, so not much has been done to calculate what amount of water is needed for a sustainable crop yield. The purpose of this study is to determine appropriate rates of irrigation for crops in southwestern Montana using micrometeorological data as boundary conditions for water- and energy-balance calculation. By making some reasonable assumptions, we will estimate what we believe is being extracted for irrigation, what is needed for irrigation, and then by finding the difference between the two we can assess the efficiency of the current irrigation methods.

The study was conducted in the Willow Creek Demonstration Watershed (WCDW) located near the Judson Mead Geologic Field Station in southwestern Montana (Figure 1). The WCDW is 414.22 km2 and approximately 12% of the landuse in the watershed is agricultural. The main crop in the WCDW is alfalfa. Most farmers use this crop not only as a cash crop but to feed their cattle as well. Alfalfa can usually be harvested twice a year, and many of the farmers in the area follow this trend. The two main drainages that are the headwaters for Willow Creek are South Fork Willow Creek and North Fork Willow Creek. South Fork Willow Creek is the larger of the two creeks, although water is extracted from both streams for irrigation.

Although the assumptions made in performing the calculations in this study probably overestimate the amount of irrigation water withdrawals from the streams in the WCDW, the results obtained from our calculations suggest that the farmers are over-irrigating their farmland, which is resulting in excess runoff. Some may argue that this runoff is eventually put back into the streams. This is a possibility, but runoff is associated with many environmental factors such as increased sedimentation and erosion as well as being contaminated by the many pesticides and herbicides used on the agricultural fields. This creates downstream contamination, which is harmful to the ecology of the stream system as well as humans and other plants and animals. Many aquatic habitats survive under very specific conditions and this lower stream discharge, as well as the increased sedimentation and pollution, is harmful to those habitats. Ongoing research is needed to more accurately model the growing season water balance of this watershed utilizing better estimates of snow melt and ground-water recharge from the streams.